The present invention relates to fluid flow valves and, more particularly, to valves for controlling the flow relationships within generally coannular ducts.
Recent studies in variable cycle gas turbine engines indicate that flow switching valves of various types will be required for flow modulation. A variable cycle engine is one which is performance optimized at more than one operating condition through the variability in geometry of specified engine components. In variable cycle engines of the gas turbofan variety wherein a pressurized fan stream bypasses a core engine stream, it is foreseeable that future requirements will necessitate that the streams be inverted, for performance optimization, somewhere along their respective duct lengths. This will require a valve which, as a minimum, will invert the flows between the coannular streams. Preferably, the valves should be able to modulate the flow rates through the ducts and, if necessary, block the flow in one or more of the ducts. Furthermore, adaptation to a gas turbine engine requires that such a valve be of lightweight construction and efficient in performance.
Simply presented, the problem is to develop a valve which receives the independent, separated flows from two concentric upstream annular ducts and directs the two independent, and still separated, flows into two downstream concentric annular ducts with the option of inverting the flow relationship at will. In its basic concept, the solution will find application not only in advanced gas turbine engines, but also in other ducted flow situations such as heating and air conditioning installations.
Previous attempts to solve similar problems have been unsuccessful and impractical due to their inherent complexity or inefficiency. It has been recognized that flow turning induces flow pressure losses which, in turn, leads to inefficiency. Thus, any potential valve system should minimize flow turning. One solution to the problem appears in U.S. Pat. No. 3,792,584 -- Klees wherein flow switching is accomplished by relative circumferential rotation of two chuted coannular duct sections such that the flows always pass straight through the interface plane. While such a scheme is theoretically optimized from the efficiency point of view, it is somewhat cumbersome to carry into practice on a gas turbofan engine since it requires circumferential rotation of large duct sections, possibly up to eight feet in diameter. An actuation system capable of rotating such a large duct and also capable of withstanding flight maneuver loadings would be heavy indeed. Alternatively, it has been suggested by Klees that while flapper valves could be employed between spaced apart sections of coannular ducts, such valves would be fully efficient in only one position where the flow remained straight between valve portions, the implication being that in any other position, the required turning and, thus, inefficiency would be so great as to be impractical. Thus, the requirement exists for practical lightweight valves which will allow for complete flow modulation and inversion in coannular ducts.